This invention relates generally to heat reflecting fenestration composites and, more particularly, to such composites of the dielectric-metal-dielectric type.
For the past few decades, heat reflecting fenestration composites have been in use to improve the energy transmission and appearance of transparent glazing used in commercial buildings, residential buildings and vehicles. The purpose for using these heat reflecting fenestration composites is generally to alter the solar energy transmission, reflection, absorption or emission of various glazing products. The most common purpose for using heat reflecting fenestration composites is to reduce solar heat gain by reflecting or absorbing as much infrared energy as possible without degrading the visible characteristics of the fenestration structure. It is usually desirable to create a transparent glazing with high to medium visible transmission and low visible reflection on both sides of the energy control sheet. It is also desirable that transmission and reflection on either side of the sheet are neutral to slightly blue-green in color. Implementing these properties in a glazing is usually done with optical thin film coatings vacuum deposited on one surface of a transparent glazing material.
A typical heat reflecting thin film product is a five layer thin film structure consisting of: dielectric/infrared reflecting metal/dielectric/infrared reflecting metal/dielectric (xe2x80x9cD/M/D/M/Dxe2x80x9d). Layer thicknesses and material choices for this design must be specifically controlled to achieve the desired optical spectrum. The dielectric layers typically chosen have high indices of refraction ranging from 1.8 to 2.5 and are often materials such as In2O3, SnO2, TiO2, Nb2O5, Ta2O5, ZnO and SiN. Designs using lower index dielectric materials such as polymers are known but less commonly made. The infrared reflecting layers typically consist of silver or alloys of silver but may be variations of gold, copper or even conductive compounds such as titanium nitride.
Different versions of this heat reflecting interference stack include designs with more or less metal/dielectric pairs. Three-layer (D/M/D) and seven-layer (D/M/D/M/D/M/D) designs are also commonly employed.
Three-layer designs are much less expensive to manufacture than five-layer designs and seven-layer designs, but have traditionally suffered from several disadvantages. First of all, three-layer designs exhibit desirable low visible reflectance levels less than 10%) only when the substrate and one surface of the thin film layer are left exposed to air. When a three-layer design is laminated so that both sides of the thin film stack are in contact with polymer or other material with indices greater than 1.0, visible reflectance often rises to levels of 13 to 25%, levels which are undesirable in many applications. The reason for this difference has to do with the optical coupling of the three-layer stack with air (having a reflective index of 1.0) versus the coupling with polymer (having a reflective index of 1.4-1.7). Effective antireflection of the reflective metal by the dielectric requires that there be a large index gradient between the dielectric and the medium it contacts on the side opposite the reflective metal. In a commercially acceptable three-layer stack, one side stack must have this wider index gradient from dielectric to surrounding medium to retain the desired lower reflectance levels. Consequently, three-layer designs have previously been found to be impractical when used in composites where it is not desirable to leave one side of the composite exposed to the air. Such designs include the many composites wherein the heat reflecting stack is sandwiched between two substrate layers.
Another disadvantage with three layer designs has to do with the difficulty in lowering the transmission of visible light through the product to below about 70%. Often, it is desirable to lower visible transmission below 70% to minimize the transmission of solar radiation. Visible transmissions between about 40% to about 60% are considered ideal for many solar control applications. At transmission is percentages in this range, heat transmission is markedly reduced without adversely affecting the overall quality of the product as a transparent window covering. In three layer designs, it is traditionally difficult to reduce the visible transmission to the 40%-60% range without causing secondary problems. For example, reducing visible transmission by increasing the thickness of the metal reflective layer almost always entails an undesirable reflectance color. On the other hand, use of the pigments or dyes to reduce transmission generally creates problem of non-uniform color or non-uniform fading of the pigments or dyes over time.
Accordingly, there is a need to modify the prior art three layer heat reflective film to avoid the above-described problems with the prior artxe2x80x94in an effective and inexpensive manner.
The present invention satisfies this need. The invention is a heat reflective composite comprising, in series, a first substrate and a heat reflective stack disposed upon the first substrate. The heat reflective stack comprises:
(1) a first interference layer;
(2) an infrared reflecting metal layer deposited onto the first interference layer;
(3) a second interference layer; and
(4) a non-infrared reflective layer deposited onto the second interference layer, the non-infrared reflective layer being composed of a material selected from the group of materials consisting of (i) metals having an index of refraction greater than about 1.0 and an extinction coefficient greater than about 2.0 and (ii) non-metals having an index of refraction greater than about 0.5 and having an extinction coefficient greater than about 0.5.
Typically, the first substrate is a pane of glass or a thin plastic material which can be applied to a pane of glass. The first and second interference layers are typically dielectric materials, such as metal oxides having indices of refraction in the visible wavelengths between about 1.8 and about 2.5. The infrared reflecting metal layer is typically silver, gold, copper or alloys thereof.
The non-infrared reflective layer is typically a layer of titanium, tantalum, niobium, chromium, molybdenum, stainless steel or nickel alloy.
The invention has been found to provide an improved heat reflective stack which is effective even when one side of the heat reflective stack is not exposed to the air. Thus, even where the heat reflective stack is sandwiched between two substrate layers, the heat reflective stack is effective in reducing heat and light transmission without undue reflectance and without adversely affecting the color of transmitted light.